Int. J. Communications, Network and System Sciences, 2012, 5, 767-773 http://dx.doi.org/10.4236/ijcns.2012.511080 Published Online November 2012 (http://www.SciRP.org/journal/ijcns) Simulation and Measurements of VSWR for Microwave Communication Systems Bexhet Kamo, Shkelzen Cakaj, Vladi Koliçi, Erida Mulla Faculty of Information Technology, Polytechnic University of Tirana, Tirana, Albania Email: [email protected], [email protected], [email protected], [email protected] Received September 5, 2012; revised October 11, 2012; accepted October 18, 2012 ABSTRACT Nowadays, microwave frequency systems, in many applications are used. Regardless of the application, all microwave communication systems are faced with transmission line matching problem, related to the load or impedance connected to them. The mismatching of microwave lines with the load connected to them generates reflected waves. Mismatching is identified by a parameter known as VSWR (Voltage Standing Wave Ratio). VSWR is a crucial parameter on deter- mining the efficiency of microwave systems. In medical application VSWR gets a specific importance. The presence of reflected waves can lead to the wrong measurement information, consequently a wrong diagnostic result interpretation applied to a specific patient. For this reason, specifically in medical applications, it is important to minimize the re- flected waves, or control the VSWR value with the high accuracy level. In this paper, the transmission line under dif- ferent matching conditions is simulated and experimented. Through simulation and experimental measurements, the VSWR for each case of connected line with the respective load is calculated and measured. Further elements either with impact or not on the VSWR value are identified. Interpretation of simulation and experimental results allows to judge about improving the VSWR, and consequently increasing the microwave transmission systems efficiency. Keywords: Microwave; Load; Impedance Matching; VSWR 1. Introduction called VSWR (Voltage Standing Wave Ratio). In the second section, the theoretical VSWR concepts are given, Generally, when a transmitter is connected through a and further the simulation and experiments are described transmission line to an antenna, or any other load con- with respective result analysis and conclusions. nected to, these elements must match to each other, in order to enable the maximum possible energy transfer 2. VSWR Concepts from the transmission line to the antenna or the load, and consequently having minimal losses. When the antenna Generally, a microwave communication system consists or load and transmission line that connects the transmit- of three main parts, [1-4]: ter from one side, and the antenna or the load to the other Radio transmitter. side, are not matched, energy is not transmitted properly. Radio receiver. A part of energy that comes from transmitter does not go Link/wireless channel between two antennas. to the antenna or the load but it is reflected back, to the The main elements of the transmitter are: oscillator, transmitter. So, a part of the energy that comes from the modulator, amplifier and antenna. On other hand the incident wave it is transmitted toward antenna, or any main elements of the receiver are: antenna, low noise other load connected to the line, but the other part of it in amplifier, selective filter, local oscillator, mixer, inter- form of waves is reflected back. Due to the presence of mediate frequency amplifier, and demodulator that gives those waves, in the transmission line, a standing wave is at the output the signal to be received. All elements in created. In microwave radio planning, it is necessary to transmitter or in receiver part are connected with each measure the voltage standing wave ratio in order to un- other using transmission lines. Thus, it is too important to derstand the mismatch level in the transmission line. assess electromagnetic waves transport over these trans- The power reflected back to the transmitter affects the mission lines and what energy is reflected back due to a performance of RF transmitter [1-4]. Standing waves are mismatch [1-4]. For further analysis it is considered a determined by the ratio of the maximum and minimum transmission line with impedance Z0 , which is con- voltage amplitude of the wave in transmission line, so nected with a load ZL as in Figure 1. Copyright © 2012 SciRes. IJCNS 768 B. KAMO ET AL. Zg tion systems, the impedance mismatch of microwave waveguide when connected to a specific load through simulation is analyzed. The load may be too close to the V g Z0 ZL characteristic impedance of a waveguide as the best case, or it might be too different compared to specific imped- ance as the worst case [6]. For simulation purposes three Figure 1. A transmission line with Z0 impedance connected scenarios are considered. The first scenario considers a with a load ZL. gateway, a transmission line and a load, as presented in Figure 2. If ZL differs from Z0 , we have a mismatch between In this case the transmission line sizes are fixed and load and line. In this case, a part of energy goes to the the load is the varying parameter. The length of the line load and a part of it is reflected back through line to the is 69mm, characteristic impedance Z0 = 50 ohm and the generator Vg . The reflection coefficient along the line is load changes from 50 ohm to 200 ohm. Figures 2(a) and defined as [1-4]: (b) respectively show the cases for 50 ohm and 100 ohm. Simulation is performed at frequency of 3 GHz. Under Z Z0 p (1) the load of 50 ohm, the same as the characteristic im- Z Z0 pedance Z0 of the line, the VSWR, is equal to 1, as or, expressed by voltage levels, the reflection coefficient shown by simulation at Figure 2(a). If the load changes is defined as the ratio of the reflected voltage (Vr) to the to 100 ohm, the VSWR is equal to 2 as shown by simula- incident voltage (Vi), as: tion at Figure 2(b), and for a load 200 ohm the VSWR goes to 4. This confirms that when the load changes and Vr p (2) it differs from the line’s characteristic impedance Z , V 0 i VSWR also changes because of reflected waves [7,8]. The incident wave and reflected wave create the so The second scenario, as in Figure 3, considers a gate- called steady wave at transmission line. The steady wave way, three transmission lines, with Z0 impedance equal to is taken as the sum of the downward wave that passes 50 ohm, a slotted line and a load of 50 ohm. In this case along the line to the load and reflected wave that comes the transmission line sizes are fixed and the parameter back. VSWR is defined as the ratio between maximum that varies is the dimension of the slotted guide. The and minimum values of the steady wave as [1-4]: slotted guide is moved in different positions. Simulations V are performed at frequency of 3 GHz. From simulation, VSWR max (3) as shown in Figure 3 the value of VSWR is equal to 1, V min and this represents the best case where the reflected If the incident and reflected voltage are in phase, these waves are too low, closed to zero. ads up, creating maximum voltage value, as: The third scenario, presented in Figure 4, considers a gateway, three transmission lines, with Z0 impedance VVV (4) max ir equal to 50 ohm, a slotted line and a load of 0 (zero) ohm or the short circuit. In this case the transmission line sizes where Vi is the r.m.s (route mean square) value of in- are fixed. The parameter that varies is dimension of slot- cident voltage and Vr is the r.m.s value of the reflected voltage. Also, ted guide. The slotted guide is moved in different posi- tion in order to get a VSWR value as shown in Figure 4. VVVmin ir (5) Simulation is performed at frequency of 3 GHz. At this From VSWR and reflection coefficient definition yields case, the value of VSWR is equal to 9.68, confirming too out the correlation between them as follows: high level of reflected waves and indicating that an im- provement of VSWR should be considered. 1 p VSWR (6) 1 p 4. VSWR Experimental Measurements In case the line is matched with the load, Z Z The laboratory scheme used for VSWR measurement in L 0 Figure 5 is given [9-12]. Transmitter generates interme- then the reflection factor is p 0 Vr 0, so VSWR = 1. This is the best scenario when the line with load it is diate frequency which can be selected as one of the four different frequency separated channels, with frequency perfectly matched [5]. separation of 27 MHz. The power level is adjustable 3. VSWR Simulation from 0 dB to - 25 dB. The transmitter uses two switches, SW1 and SW2, for Considering the importance of VSWR for communica- channel frequency selection, as presented in Table 1. Copyright © 2012 SciRes. IJCNS B. KAMO ET AL. 769 (a) (b) Figure 2. The first VSWR simulation scenario (Varying load). (a) Case for 50 ohm load; (b) Case for 100 ohm load. Table 1. The frequencies for four different channels. = 8350 MHz, which is synthesized with PLL, while gen- erating signals at the output of the mixer. After mixing CH 1 2 3 4 with intermediate frequency, is generated signal at ra- SW1 1 0 1 0 diofrequency band as: SW2 1 1 0 0 fRFIFLO ff (8) Δf 0 27 54 81 For four known intermediate frequencies and local oscillator frequency, RF frequencies are given in Table Frequency is determined as: 2.